Intersymbol interference (ISI) presents the primary impediment to the reliable transmission of digital information over highly band-limited channels. High-speed digital data transmission over telephone lines and high-density digital recording on magnetic medium are good examples of the digital transmission in the presence of ISI. ISI also arises in a certain modulation scheme called partial response signaling wherein some portions of ISI are allowed or even enhanced for the purpose of spectral shaping.
When ISI is present, the signal energy available for detection spreads out into many symbol intervals. This motivates the use of detection schemes which make decisions based on a series of observation samples. In the presence of large ISI, the performance of any detector making decisions on a symbol-by-symbol basis is expected to fall well below the optimum performance.
One of the most prominent detection schemes that make decisions based on observations through many symbol intervals is the Viterbi detector (VA), which finds the input sequence that "best matches" the observation sequence in a recursive fashion. It is well known that the VA is effectively the optimum detector for the channels with ISI for any reasonable criterion. A drawback of the VA is its complexity, which grows exponentially with the extent of ISI. For many real channels the requirement on the data rate or linear storage density inevitably introduces significant ISI. The processing limitation and/or implementation cost prohibit the use of the VA for these channels. Another disadvantage associated with this scheme is the inherent variable delay between the time that the observed signal is fed into the detector and the time that the detected bits become available. This can hamper the timing recovery.
The present invention involves a sequence detection algorithm, with substantially reduced cost and decision delay but without a significant performance loss compared to the optimum detector. It is accomplished without violating the speed requirement imposed by data rates of modern digital transmission or storage systems. Therefore, the fast processing capability is another essential factor to be considered in developing practical sequence detectors.